Grounding switch method and apparatus

ABSTRACT

A grounding switch is described which operates properly even in the presence of negative voltages on a signal line. The grounding switch uses isolated field effect transistors that have their substrates tied to different voltages. The isolated field effect transistor has a gate voltage and substrate voltage which can be pulled down to a negative voltage when the signal line has a negative voltage allowing the switch to remain open even with a negative voltage.

TECHNICAL FIELD

This invention relates generally to the electronic switches andspecifically to grounding switches that operate in the presence ofnegative voltage signals.

BACKGROUND ART

Typically, grounding switches, often comprising a single field effecttransistor (FET), are used to ground an input or an output to a zero orground voltage. In audio applications this can be used to prevent audioartifacts such as audible pops. For example, to prevent an undesirableaudible pop a grounding switch can be used to ground an input to anaudio circuit during an initialization phase before the audio circuit isequipped to handle an input signal. The switch can release the inputfrom ground once the audio circuit has been initialized. Anotherapplication is where a grounding switch is used to ground an outputduring the startup of an audio circuit, when the audio circuit mayproduce glitches in the output leading to an audible pop.

SUMMARY OF INVENTION

A grounding switch is described which operates properly in the presenceof negative voltages on a signal line. In one embodiment, the switchcomprises an n-channel field effect transistor (NFET) with an isolatedsubstrate which allows the substrate near the NFET to have a differentpotential than the substrate around the other circuitry in the groundingswitch. A pull down element is used to turn this NFET off.

In one embodiment, the switch comprises a pull up circuit and thesubstrate of the NFET is coupled to a negative supply voltage. When thecontrol signal is low, the pull up circuit is inactive and the pull downelement pulls the gate of the NFET down to the negative supply voltagecausing the NFET to turn off even in the presence of a signal with anegative voltage on its source or drain. A second NFET, which does nothave to have an isolated substrate, can be added in series with thefirst NFET to prevent damage to the first NFET due to large voltageswings. In one variant, the pull down element comprises a singleresistor.

In another embodiment where a negative supply voltage is not available,the substrate of the isolated NFET is tied to the signal line. Theswitch comprises a second NFET, which need not have a separate substrateconnection, in series with the isolated NFET. When the control input ishigh, both NFETs turn on, turning the switch on. When the control inputis low, the isolated NFET switches off when the signal has a negativevoltage and the second NFET switches off when the signal has a positivevoltage, thus switching the grounding switch off regardless of thesignal voltage.

In one embodiment the pull down element comprises a circuit havinganother isolated NFET where the drain and substrate are connected to thesignal. In another embodiment, the pull-up circuit comprises a p-channelfield effect transistor (PFET), optionally a second PFET, and aninverter.

In another embodiment, the grounding switch is a circuit comprising twotransistors in series operable to turn on when the control input ishigh. In operation, the first transistor is turned off when the controlinput is low and the signal voltage is positive, and the secondtransistor is turned off when the control input is low and the signalvoltage is negative. The second transistor can be turned on by pullingthe gate of the transistor to the positive supply voltage and can beturned off by pulling the gate down to the negative supply voltage whilemaintaining its substrate at the negative supply voltage. Alternately,the second transistor can be turned off when the signal voltage isnegative by pulling the gate down to the signal voltage whilemaintaining its substrate at the signal voltage.

In one embodiment, the grounding switch is used in an audio driver tosuppress audible pops. The audio driver can be used to suppress anundesired audio artifact in many electronic devices including but notlimited to personal computer sound cards, voice-over-IP telephones,cellular telephones, digital picture frames, universal serial busheadsets, televisions, video game consoles, MP3 players and Bluetoothheadsets.

Other systems, methods, features, and advantages of the presentdisclosure will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present disclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1A shows an embodiment of a system employing a grounding switch totie a single ended input to ground;

FIG. 1B shows an embodiment of a system employing a grounding switch totie a single ended output to ground;

FIGS. 1C and 1D shows differential analogs of the systems described inFIGS. 1A and 1B;

FIG. 2 shows an embodiment of a grounding switch comprising a singleNFET;

FIG. 3 shows an embodiment of a grounding switch which can toleratenegative voltages seen at the signal line;

FIG. 4 shows an embodiment of a grounding switch which can toleratenegative voltages on a signal line without a negative reference voltage;

FIG. 5 shows an embodiment of a grounding switch;

FIG. 6 shows another embodiment of a grounding switch; and

FIG. 7 an embodiment of audio driver employing a grounding switch tosuppress audible pop during the power up and power down of the audiodriver.

DETAILED DESCRIPTION

A detailed description of embodiments of the present invention ispresented below. While the disclosure will be described in connectionwith these drawings, there is no intent to limit it to the embodiment orembodiments disclosed herein. On the contrary, the intent is to coverall alternatives, modifications and equivalents included within thespirit and scope of the disclosure as defined by the appended claims.

FIG. 1A shows an embodiment of a system employing a grounding switch totie a single ended input to ground. System 102 can be any system whichreceives a single ended input signal. Grounding switch 104 ties theinput to ground when closed. An example of such a system is an audiodriver which employs grounding switch 104 to zero the signal duringpower up and power down to prevent the occurrence of a pop sound.

FIG. 1B shows an embodiment of a system employing a grounding switch totie a single ended output to ground. System 102 can be any system whichproduces a single ended output signal. Grounding switch 106 ties theoutput to ground when closed. As an example, an audio driver can employgrounding switch 106 to zero the output signal during power up and powerdown to prevent the occurrence of a pop sound.

FIGS. 1C and 1D shows differential analogs of the systems described inFIGS. 1A and 1B. In particular, FIG. 1C shows an embodiment of system112 employing switch 114 to tie the two differential inputs together.Though the switch does not specifically tie a signal to ground, for thepurposes of this disclosure, a grounding switch can also be used to zeroa differential signal by tying the differential signal lines together.System 112 has a differential input which is zeroed by switch 114 whenswitch 114 is closed. Similarly, FIG. 1D shows an embodiment of system116 employing switch 118 to tie the two differential outputs together.System 116 has a differential output which is zeroed by switch 118 whenswitch 118 is closed.

FIG. 2 shows an embodiment of a grounding switch comprising a singleNFET. The gate of NFET 202 is connected to a control input, the drain ofNFET 202 is connected to signal line 204 and the source is connected toa ground potential. If the switch is used to zero a differential input,signal line 204 is one of the signal lines (e.g., the positive signalline) and the drain is connected to the other signal line (e.g., thenegative signal line). When the control input is tied to the positivesupply voltage the switch turns on. When the control input is tied toground, NFET 202 acts as a reverse bias diode and the switch turns off.However, if signal line 204 permits a negative voltage, and NFET 202 isa standard NFET that has a grounded substrate, NFET 202 acts as aforward bias diode when the gate is grounded causing the switch toconduct even though it is supposed to be off. For this reason use of asingle NFET is undesirable in many applications, such as audioapplications where the voltage can swing in both a positive and negativedirection.

FIG. 3 shows an embodiment of a grounding switch which can toleratenegative voltages seen at signal line 204. Grounding switch 300comprises controllable pull up circuit 304, isolated NFET 306, optionalNFET 308, pull down element 310, and isolated NFET 312 Controllablepull-up circuit 304 is responsive to a control signal and pulls thevoltage of the gate of isolated NFET 306 to the positive supply voltage(shown as V_(DD)) when the control signal is high and provides highimpedance when the control signal is low. Isolated NFET 306 iselectrically isolated from the substrate and more specifically, itsp-well is isolated. This allows the p-well surrounding the NFET 306 tobe tied to a different “substrate voltage” from the rest of thecircuitry. In this case, NFET 306 has an “isolated substrate connection”tied to a negative supply voltage (for example −V_(DD) shown in thefigure), represented by a fourth connection to the usually threeconnection NFET symbol. Many techniques exist to fabricate isolated FETsincluding deep n-well fabrication. Finally pull down element 310 whichcan be a resistor is tied to the negative supply voltage.

When the control input to grounding switch 300 is tied to the supplyvoltage, NFET 308 is turned on. In addition, pull up circuit 304 pullsup the gate voltage of NFET 306 so it turns on as well, thus turningswitch 300 on. This pulls the voltage on signal line 204 to ground.

When the control input to grounding switch 300 is tied to ground,pull-up circuit 304 is deactivated and pull-down element 310 can pullthe voltage down to the negative supply voltage which causes NFET 306 toturn off, even in the presence of a negative voltage on signal line 204.However, if the voltage on signal line 204 is positive such as V_(DD)the gate to drain voltage of NFET 306 as a result of the pull downelement would be 2 V_(DD), which can exceed the tolerance of NFET 306.Therefore, NFET 312 is included to protect NFET 306. Because the gate ofNFET 306 does not need to pull down to −V_(DD), NFET 312 is used tocounteract pull down element 310. In fact, when the voltage on signalline 204 is positive, NFET 312 permits a current to flow which allowsthe voltage on the gate to NFET 306 to rise so that the gate to drainvoltage can be within the tolerance of the technology. Because of thepotential for a negative source voltage on NFET 312, NFET 312 has asubstrate voltage coupled to the negative supply line. NFET 312 OptionalNFET 308 can be included to protect NFET 306 from excessive voltagesthat can occur especially if the signal line swings between the extremepositive and negative voltages, when complementarymetal-oxide-semiconductor (CMOS) technology is used. In othertechnologies or even other CMOS technologies with different designrules, NFET 308 can be omitted. In the present embodiment, the switchoperates only so long as the voltage on signal line 204 remains greaterthan the negative supply voltage.

FIG. 4 shows an embodiment of a grounding switch which can toleratenegative voltages on a signal line without the need for a negativesupply voltage. Grounding switch 400 comprises controllable pull-upcircuit 402, pull-down element 404, isolated NFET 406 and NFET 408.Controllable pull-up circuit 402 functions in a manner similar to thatdescribe for circuit 302 above. NFET 406 is isolated in the same manneras that described for NFET 306 above however the substrate voltage istied to the drain voltage.

When a positive supply voltage is applied to the control signal, NFET404 turns on. In addition, pull-up circuit 402 pulls up the gate voltageon NFET 406 causing NFET 406 to turn on, thus turning the switch on.When the control signal is grounded and the voltage on signal line 204is positive, NFET 408 is turned off. Since NFET 408 is in series withNFET 406, the switch is turned off. When the control signal is groundedand the voltage on signal line 204 is negative, pull-up circuit 402 isleft in a high impedance state, allowing pull-down element 404 to pulldown the gate voltage of NFET 406 down to the voltage of signal line 204which is also the drain voltage of NFET 406, causing NFET 406 to turnoff. Since NFET 408 and NFET 406 are in series, the switch is turnedoff.

FIG. 5 shows an embodiment of grounding switch 500. Pull up circuit 402includes an inverter 502 and PFET 504, and pull-down element 404includes isolated NFET 506. When a control signal is high, inverter 502grounds the gate of PFET 504 which turns on PFET 504, causing a positivegate voltage at NFET 406 which turns NFET 406 on. Because both NFET 406and NFET 408 are turned on, the switch is turned on. When the controlsignal is grounded, inverter 502 imposes a positive supply voltage onthe gate of PFET 504 turning the PFET off effectively disconnecting thepull-up circuit 402 from NFET 406. With the pull-up circuitdisconnected, NFET 506 can manipulate the voltage on the gate of NFET406. When the voltage on signal line 204 is positive, the drain andsubstrate of NFET 506 is positive, NFET 506 acts as a forward biaseddiode between the source and the substrate connections. As a result,NFET 506 pulls up the gate of NFET 406. Likewise, because the drain andsubstrate of NFET 406 is also positive, NFET 406 acts as a forwardbiased diode and pulls up on the drain of NFET 408. However, because thecontrol line is low, NFET 408 is turned off, so the switch is turnedoff. When the voltage on signal line 204 is negative, the role of thesource and drain are essentially reversed. The drain to gate voltage isnegative, but can be viewed as a positive gate-to-source voltage of NFET506 where the source and drain are reversed. This causes NFET 506 toturn on which pulls down (because the signal line is negative it is apull down rather than a pull up) the gate voltage of NFET 406 to thesignal line 204 voltage. Because the drain (which now functions as asource) voltage and the gate voltage are made equal by NFET 506, NFET406 turns off, turning the switch off. One advantage in this embodimentof NFET 506 as the pull-up element over a resistor described in FIG. 3is that NFET 506 only draws current in the specific case where thevoltage on signal line 204 is negative, whereas a resistor would drawcurrent all the time.

FIG. 6 shows another embodiment of a grounding switch. In thisembodiment, the voltage of the signal line swings between V_(DD) and−V_(DD), Thus, the total voltage between the source and drain of PFET502 could be 2V_(DD) which can be outside the specifications of thetransistor technology. This can potentially cause PFET 502 to operateout of specification, which can damage PFET 502. To address this problemPFET 602 is added to pull up circuit 402, by adding PFET 602 togrounding switch 600, the worst case voltage between the source anddrain of each transistor is V_(DD).

FIG. 7 an embodiment of audio driver employing a grounding switch tosuppress audible pop during the power up and power down of the audiodriver. The audio driver is shown comprising a two stage audioamplifier. For a digital audio driver, it can further comprise a digitalto analog converter (not shown) as well as other audio processingcomponents. In the example shown, the two stage audio amplifiercomprises amplifier stage 702, and output stage 720. Output stage 720comprises output driver 708, capacitor 704, resistor 706 and groundingswitch 600. Capacitor 704 and resistor 706 are used to provide stabilityto the two stage amplifier. Grounding switch 600 is used to ground theoutput of output driver 708 during power up and power down so that anaudible pop is not heard by the listener. Control signal ctrl is sethigh during power up and power down so grounding switch 600 is closed.Once the amplifier stage powers up and has settled into an operationalmode. The grounding switch is opened by setting ctrl low and the audiosignal is allowed to pass externally where a listener can hear it. Itshould be noted that while grounding switch 600 is used as an example,other embodiments, including any of the grounding switches describedabove, can also be used. Furthermore, since voltages are relative, thegrounding switches described herein can also be used to tie differentialinputs or outputs together.

Audio drivers such as that described are integral to a wide variety ofelectronic devices including but not limited to personal computer soundcards, voice-over-IP telephones, cellular telephones, digital pictureframes, universal serial bus headsets, televisions, video game consoles,MP3 players and Bluetooth headsets.

It should be emphasized that the above-described embodiments are merelyexamples of possible implementations. Many variations and modificationsmay be made to the above-described embodiments without departing fromthe principles of the present disclosure. All such modifications andvariations are intended to be included herein within the scope of thisdisclosure and protected by the following claims.

What is claimed:
 1. A switch connecting a first signal line and a secondsignal line comprising: a first isolated n-channel field effecttransistor (NFET) with an isolated substrate connection, having a gateand a drain, wherein the drain is coupled to the first signal line; asecond NFET having a gate coupled to a control signal and a sourcecoupled to the second signal line, wherein the second NFET is in serieswith the first isolated NFET; a controlled pull-up circuit operable toreceive the control signal, the controlled pull-up circuit coupled tothe gate of the first isolated NFET; and a pull-down circuit coupled tothe gate of the first isolated NFET; wherein the pull-up circuit pullsup the gate of the first isolated NFET to a positive supply voltage whenthe control signal is in a high state; and wherein the pull-down circuitpulls the gate of the first isolated NFET down to a negative supplyvoltage or the voltage of the first signal line when the control signalis in a low state.
 2. The switch of claim 1 wherein the isolatedsubstrate connection of the first isolated NFET is connected to anegative supply voltage and the pull-down circuit is coupled to thenegative supply voltage.
 3. The switch of claim 2, wherein the pull downcircuit comprises a resistor.
 4. The switch of claim 1 wherein theisolated substrate connection of the first isolated NFET is connected tothe drain of the first isolated NFET and the pull-down circuit iscoupled to the drain of the first isolated NFET.
 5. The switch of claim4, wherein the pull down circuit comprises a third isolated NFET havinga drain and an isolated substrate connection, wherein the isolatedsubstrate connection of the third isolated NFET is connected to thedrain of the third isolated NFET.
 6. The switch of claim 1, wherein thepull-up circuit comprises a p-channel field effect transistor (PFET)including a gate and an inverter coupled to the gate of the PFET.
 7. Theswitch of claim 6, wherein the pull-up circuit further comprises asecond PFET.
 8. In an electronic device, an audio driver comprising: anamplifier stage; an output stage comprising an output driver, a resistorand a capacitor; and a grounding switch; wherein the amplifier stage iscoupled to the output stage and the grounding switch is coupled to anoutput of the output stage; and wherein the grounding switch comprises:a first isolated NFET with an isolated substrate connection, having agate and a drain; a second NFET having a gate coupled to a controlsignal, wherein the second NFET is in series with the first isolatedNFET; a controlled pull-up circuit operable to receive a control signal,coupled to the gate of the first isolated NFET; and a pull-down circuitcoupled to the gate of the first isolated NFET; wherein the pull-upcircuit pulls up the gate of the first isolated NFET to a positivesupply voltage when the control signal is in a high state; and whereinthe pull-down circuit pulls the gate of the first isolated NFET down toa negative supply voltage or the voltage of the first signal line whenthe control signal is in a low state.
 9. The audio driver of claim 8wherein the isolated substrate connection of the first isolated NFET isconnected to a negative supply voltage and the pull-down circuit iscoupled to the negative supply voltage.
 10. The audio driver of claim 9,wherein the pull down circuit comprises a resistor.
 11. The audio driverof claim 8 wherein the isolated substrate connection of the firstisolated NFET is connected to the drain of the first isolated NFET andthe pull-down circuit is coupled to the drain of the first isolatedNFET.
 12. The audio driver of claim 11, wherein the pull down circuitcomprises a third isolated NFET having a drain and an isolated substrateconnection, wherein the isolated substrate connection of the thirdisolated NFET is connected to the drain of the third isolated NFET. 13.The audio driver of claim 8, wherein the pull-up circuit comprises aPFET coupled to an inverter.
 14. The audio driver of claim 13, whereinthe pull-up circuit further comprises a second PFET.
 15. The audiodriver of claim 8 wherein the electronic device is one of a personalcomputer sound card, a voice-over-IP telephone, a cellular telephone, adigital picture frame, a universal serial bus headset, a television, avideo game console, an MP3 player or a Bluetooth headset.